Pollution control devices are universally employed on motor vehicles to control atmospheric pollution. Two types of devices are currently in widespread use—catalytic converters and diesel particulate filters or traps. Catalytic converters contain a catalyst, which is typically coated on a monolithic structure mounted within a metallic housing. The monolithic structures are typically ceramic, although metal monoliths have also been used. The catalyst oxidizes carbon monoxide and hydrocarbons and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution.
Diesel particulate filters or traps are typically wall flow filters, which have honeycombed, monolithic structures typically made from porous crystalline ceramic materials. Alternate cells of the honeycombed structure are typically plugged such that exhaust gas enters in one cell and is forced through the porous wall to an adjacent cell where it can exit the structure. In this way, the small soot particles that are present in diesel exhaust gas are collected.
The monoliths and in particular the ceramic pollution control monoliths, used in pollution control devices are fragile and susceptible to vibration or shock damage and breakage. They have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. This means that as the pollution control device is heated the gap between the inside peripheral wall of the housing and the outer wall of the monolith increases. Even though the metallic housing undergoes a smaller temperature change due to the insulating effect of the mat, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling occurs hundreds of times during the life and use of the pollution control device.
To avoid damage to the ceramic monoliths from for example road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and metal housing (thereby bypassing the catalyst), mounting mats are disposed between the ceramic monolith and the metal housing. These mats must exert sufficient pressure to hold the monolith in place over the desired temperature range but not so much pressure as to damage the ceramic monolith.
Recently, there has been a trend towards increasing the number of cells that make up the pollution control monolith per unit area and reducing the wall thickness of the cells. Such pollution control monoliths are known as thin wall or ultra-thin wall monoliths and typically have between 400 and 1200 cells per square inch (cpsi) and a wall thickness of not more than 5 mils, i.e., 0.005 inch (0.127 mm). Because of the reduced wall thickness, these monoliths are even more susceptible to damage and accordingly, the mounting mats for mounting such monoliths are subject to more stringent requirements.
Many mounting mats have been described in the art. Known mounting mats include intumescent sheet materials comprised of ceramic fibers, intumescent materials and organic and/or inorganic binders. Intumescent sheet materials useful for mounting a catalytic converter in a housing are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.) U.S. Pat. No. 5,151,253 (Merry et al.) U.S. Pat. No. 5,250,269 (Langer) and U.S. Pat. No. 5,736,109 (Howorth et al.). Intumescent mounting mats have the disadvantage that they may exert too much pressure on the pollution control monolith during use when the pollution control monolith heats up. As a result, intumescent mounting mats are less suitable for mounting thin wall and ultra-thin wall monoliths.
U.S. Pat. No. 5,290,522 describes a catalytic converter having a non-woven, mounting mat comprising at least 60% by weight shot-free high strength magnesium aluminosilicate glass fibers having a diameter greater than 5 micrometers. Such mounting mat may however not have sufficient holding strength to satisfactorily mount thin wall and ultra-thin wall monoliths at high temperature and protect them against shock and damage.
U.S. Pat. No. 5,380,580 discloses a non-woven mat of physically entangled shot-free ceramic oxide fibers. The mat is taught to be useful as filter medium, mounting mat and sound or thermal insulation. In one of the examples, a non-woven mat is disclosed that comprises a layer of a polycrystalline ceramic fiber and a layer of glass fiber. The mat is apparently meant for thermal insulation and would not be readily suitable as a mounting mat for mounting a pollution control monolith in a pollution control device. Also, there is no teaching as to how such a bilayer mat is to be used as a mounting mat.
Non-intumescent mats comprised of polycrystalline ceramic fibers and binder have been proposed for mounting so-called ultra thin-wall monoliths. Examples of non-intumescent mats are described in, for example, U.S. Pat. No. 4,011,651 (Bradbury et al.), U.S. Pat. No. 4,929,429 (Merry), U.S. Pat. No. 5,028,397 (Merry), U.S. Pat. No. 5,996,228 (Shoji et al.), and U.S. Pat. No. 5,580,532 (Robinson et al.). Polycrystalline fibers are typically formed through a sol-gel process as described in, for example, U.S. Pat. No. 3,760,049 whereas other ceramic fibers are typically melt formed. Unfortunately, polycrystalline fibers are much more expensive than melt formed ceramic fibers such as ceramic glass fibers and as a result mounting mats of polycrystalline fibers are often prohibitively expensive.
Accordingly, it is desired to find further mounting mats suitable for mounting pollution control monoliths in a pollution control device and in particular such mounting mats that can be used to mount thin wall or ultra-thin wall monoliths. Preferably such mounting mats provide a good holding force of the monolith particularly at high temperature without exerting too much pressure that could cause damage to the monolith. Preferably, the mounting mat can be produced at low cost and is preferably also environmentally friendly.